1. FIELD OF THE INVENTION
This invention relates to a planar array antenna including radiation elements in the form of microstrips, and more particularly to an antenna of the kind described above which is suitable for use in, for example, a Doppler radar which detects the ground speed of a body moving relative to the ground.
2. DESCRIPTION OF THE RELATED ART
FIG. 12 shows the directivity of a prior art planar array antenna, and it will be seen in FIG. 12 that undesirable side lobes SL tend to appear besides a main lobe ML of the directivity. FIG. 13 is a schematic plan view of another prior art planar array antenna in which distances d.sub.1, d.sub.2, . . . , d.sub.m, . . . , d.sub.N between the centerline of a continuous feeder line 6 and the centers of radiation elements 81 to 85 are suitably selected respectively to provide a desired amplitude distribution, so that undesirable side lobes of the directivity of the antenna can be set at a desired level. In FIG. 13, reference numerals 9, 20 and 21 designate a substrate made of a dielectric material, a transmitter acting as a signal source, and a matching load, respectively.
However, in the case of the prior art antenna structure, the power coupling coefficient (the ratio between the power radiated into the space as radio waves and the total input power) is as small as of the order of 0 to 0.4. Therefore, when such a planar array antenna has a small number of radiation elements or it is designed to have side lobes of a low level, it is necessary to use a matching load 21 to thereby consume a part of the power as shown in FIG. 13. As a result, the radiation efficiency is inevitably reduced.
On the other hand, when it is intended to raise the power coupling coefficient, the degree of matching between the feeder line and the radiation elements is degraded, which rendered the prior art planar array antenna design difficult to attain the above object.